Roundness measuring machines are used to measure a contour of a workpiece.
To measure a contour of a workpiece by a roundness measuring machine, the workpiece is mounted on a rotary table and a probe is brought into contact with a circumferential surface of the workpiece rotated along with the table (see Patent Literature 1: JP 2016-065751 A).
To perform an accurate measurement, the roundness measuring machine may perform an operation control so that the rotation speed of the workpiece and the table falls within a predetermined range, when the probe is brought into contact with the circumferential surface of the workpiece to perform measurement.
In measuring the circumferential surface of the workpiece by the above roundness measuring machine, the workpiece needs to be rotated at a predetermined speed suitable for measurement. A driving force is thus applied to the table to accelerate the workpiece from a resting state to a desired rotation speed.
Incidentally, each workpiece has its own inertia moment. Thus, even when the same driving force is applied to the table, a time (acceleration time) elapsed before the rotation speed of each workpiece reaches the desired range is different. In other words, a workpiece having a large inertia moment requires a long acceleration time, while a workpiece having a small inertia moment requires a short acceleration time.
Such an acceleration time should preferably be shortened as much as possible in terms of operating efficiency because, if so, a measurement operation can be started soon. However, when the acceleration time is extremely short, the workpiece is unlikely to be appropriately measured at the desired rotation speed due to an insufficient acceleration of the workpiece.
It should be noted that a workpiece having a large inertia moment can be sufficiently accelerated in a short time by increasing the driving force for the table. However, a powerful driving mechanism is unsuitable for enhancing the accuracy because such a mechanism should be unnecessary excess equipment for a measurement state with a constant rotation speed and may mechanistically impose an excessive load on the measuring device as a whole. Accordingly, the driving mechanism for the table has not been powered up more than necessary.
Further, although an acceleration time shortened as much as possible is also favorable for an arrangement for controlling the acceleration of the table, an acceleration time suitable for the inertia moment of the workpiece mounted on the table is not automatically determinable. If the determined acceleration time is shorter than an acceleration time suitable for the inertia moment of the workpiece, the table, which is affected by a large inertia moment, is controlled to be rapidly accelerated, causing a problem to the operation of a motor that drives the table.
To deal with the above problems related to the acceleration time, a typical roundness measuring machine uses a sufficiently long acceleration time determined in accordance with the inertia moment of the largest workpiece (i.e., one of measurable workpieces with the largest weight and diameter).
Specifically, the acceleration time in such a typical roundness measuring machine is set long to ensure that the acceleration time becomes sufficient for even a workpiece having a large inertia moment to reach a desired rotation speed necessary for measurement without causing a problem to the operation of the motor. In other words, it has been considered that performing a reliable measurement has priority over operating efficiency.
Further, similar problems may be caused in relation to a time (deceleration time) elapsed before the workpiece is decelerated to the resting state after a brake is applied to the table with the workpiece being rotated.
In other words, when a brake is applied to a workpiece having a large inertia moment in a short time, a load on a mechanistical portion is disadvantageously increased. Additionally, an angular position of the table is inaccurately determined when a brake is applied to a workpiece having a large inertia moment in a short time. Accordingly, the deceleration time has also been set sufficiently long.
An acceleration/deceleration time (acceleration time and deceleration time) of the table of the typical roundness measuring machine has thus been set sufficiently long to allow a workpiece having a large inertia moment not to be forcibly accelerated and decelerated with a high accuracy.
However, such a sufficiently long acceleration/deceleration time is longer than actually required acceleration/deceleration times for almost all the various workpieces. This results in increasing the total time required to measure the workpieces and, consequently, in lowering a processing efficiency of the roundness measuring machine per unit time.